How to retrieve multiple values matching a single SQL id

I'm trying to gather multiple "Class ID's" for a single "Teacher" using an SQL query inside a C# Web app. I've been able to successfully link the tables in the query but I'm only receiving one class taught per teacher, even when there are multiple classes taught by one teacher in the database.

Here's my code to generate the SQL query and post the information:

public Teacher FindTeacher(int TeacherId)
        {
            MySqlConnection Conn = School.AccessDatabase();
            
            Conn.Open();

            MySqlCommand cmd = Conn.CreateCommand();

            cmd.CommandText = "Select * from Teachers, Classes where teachers.teacherid = " + TeacherId;

            MySqlDataReader ResultSet = cmd.ExecuteReader();

            Teacher SelectedTeacher = new Teacher();

            while (ResultSet.Read())
            {
                int Id = Convert.ToInt32(ResultSet["teacherid"]);
                string TeacherFName = ResultSet["teacherfname"].ToString();
                string TeacherLName = ResultSet["teacherlname"].ToString();
                string TaughtClassName = ResultSet["classname"].ToString();
                string TaughtClassCode = ResultSet["classcode"].ToString();

                SelectedTeacher.TeacherId = Id;
                SelectedTeacher.TeacherFName = TeacherFName;
                SelectedTeacher.TeacherLName = TeacherLName;
                SelectedTeacher.TaughtClassCode = TaughtClassCode;
                SelectedTeacher.TaughtClassName = TaughtClassName;
            }

EDIT: Thank you for the help so far, I'm quite new to this So I appreciate the assistance. I've changed the syntax to an INNER JOIN but I'm still not getting the desired output; I want the output to be like this: "Mr Smith teaches Class A and Class B" where "Class A" and "Class B" are both fetched from the database. Here's my updated code:

 //Set up and define query for DB
            MySqlCommand cmd = Conn.CreateCommand();
            cmd.CommandText = "Select * from Teachers Join Classes on teachers.teacherid = classes.teacherid where teachers.teacherid=" + TeacherId;

            //Collect query result in a variable 
            MySqlDataReader ResultSet = cmd.ExecuteReader();

            //Create a variable in which to store the current teacher
            Teacher SelectedTeacher = new Teacher();

            //go through each row of the query result
            while (ResultSet.Read())
            {
                int Id = Convert.ToInt32(ResultSet["teacherid"]);
                string TeacherFName = ResultSet["teacherfname"].ToString();
                string TeacherLName = ResultSet["teacherlname"].ToString();
                string TaughtClassName = ResultSet["classname"].ToString();
                string TaughtClassCode = ResultSet["classcode"].ToString();

                SelectedTeacher.TeacherId = Id;
                SelectedTeacher.TeacherFName = TeacherFName;
                SelectedTeacher.TeacherLName = TeacherLName;
                SelectedTeacher.TaughtClassCode = TaughtClassCode;
                SelectedTeacher.TaughtClassName = TaughtClassName;
            }